The prior art refrigeration system is shown in FIG. 8, where a compressor 801, a condenser 803, a receiver 805, an expansion valve 809, and an evaporator 813 are connected in turn. A vapor pressure-adjusting valve 815 is disposed between the evaporator 813 and the compressor 801 to maintain the pressure of gaseous refrigerant inside the evaporator 813 constant. When the cooling load drops and the vapor pressure inside the evaporator 813 falls, the adjusting valve 815 senses the pressure drop to reduce the flow of refrigerant from the evaporator 813 to the compressor 801. Thus the vapor pressure inside the evaporator 813 is prevented from dropping.
FIG. 9 shows a Mollier chart for illustrating the condition in which the vapor pressure-adjusting valve 815 operates to reduce the flow of refrigerant from the evaporator 813 to the compressor 801. At point A, the compressor 801 is absorbing refrigerant. At point B, refrigerant is expelled from the compressor 801 but is not yet drawn into the condenser 803. At point C, refrigerant flowing out of the condenser 803 is directed toward the expansion valve 809. At point D, refrigerant discharged from the expansion valve flows toward the evaporator 813. At point E, refrigerant flows out of the evaporator 813. The operation of the adjusting valve 815 is given by the lines defined by these points E-A.
In the aforementioned refrigeration cycle, when the vapor pressure-adjusting valve 815 is working to reduce the flow of refrigerant which is forced out of the evaporator 813 toward the compressor 801, the refrigerant flowing out of the valve 815 toward the compressor 801 is overheated gas (point A in FIG. 9). The valve 815 also acts to reduce the flow of refrigerant drawn into the compressor 801. Ideally all the sliding portions inside the compressor 801 should be cooled by a sufficient amount of refrigerant admitted into it. However, when the valve 815 is working as mentioned above, the flow of refrigerant drawn into the compressor 801 is low. Further, this refrigerant is overheated gas. Therefore, it is impossible to cool the compressor 801 sufficiently. Hence, the seal may deteriorate, or the sliding portions may seize because of excessive heat.